1. Field of the Invention
The present invention relates to a vibration wave driving apparatus such as a vibration wave motor using a vibration, which an elastic member is caused to generate, as a driving force and, more specifically, to a structure of an electro-mechanical energy conversion element for causing the elastic member to generate a vibration.
2. Related Background Art
FIG. 13 is a sectional view of a vibration wave motor.
In the figure, reference numeral 40 denotes a vibration member, which is constituted of an elastic member 10 made of metal or the like, a piezoelectric element 20 functioning as an electro-mechanical energy conversion element and a frictional member 30. The piezoelectric element 20 is fixed to one side of the elastic member 10 and the frictional member 30 is fixed to the other side.
The vibration member 40 is fixed to a housing 50 and a case 110 is fixed to the housing 50. Bearings 120a and 120b are fixed to the case 110 and the housing 50. The bearings 120a and 120b rotatably support a rotary shaft 100.
A rotor 60 having the rotary shaft 100 as a center is pressurized to contact the frictional member 30 of the vibration member 40. The rotor 60 is pressurized toward the vibration member 40 by a pressurizing mechanism 90 consisting of a pressurizing spring 70 and a spring bracket 80. The spring bracket 80 is fixed to the rotary shaft 100 and the rotor 60, the pressurizing spring 70, the spring bracket 80 and the rotary shaft 100 integrally rotate.
The piezoelectric element 20 used for a vibration wave motor of a shape shown in FIG. 13 has a structure in which electrode films are provided on both sides of one piezoelectric ceramic of a circular plate shape. It is assumed that the electrode provided on one side is an electrode for applying a voltage from a power feeding substrate and the electrode provided on the other side is an electrode for ground. When an alternating voltage is applied to the piezoelectric element 20, standing wave vibrations of different phases A and B are composited to generate a travelling wave on the surface of the elastic member 10.
FIG. 14A shows an example of a conventional electrode pattern for inputting an alternating voltage. Given that a wavelength of a travelling wave is λ, a plurality of electrodes for the A phase that are alternately polarized in opposite directions at a λ/2 pitch in their thickness direction and a plurality of electrodes for the B phase that are λ/4 apart from the electrodes for the A phase and are alternately polarized in opposite directions at a λ/2 pitch in their thickness direction are formed. FIG. 14B shows an example of an electrode pattern for ground, in which a circular electrode along a shape of a piezoelectric element is formed.
In the electrode pattern shown in FIG. 14A, the electrodes for the A phase are formed on one side of the circle and the electrodes for the B phase are formed on the other side. A vibration generated in the electrodes for the A phase has a smaller amplitude as the vibration travels farther from the electrodes. A vibration generated in the electrodes for the B phase has a smaller amplitude as the vibration travels farther from the electrodes.
This state is shown in FIGS. 15A to 15C. In the figures, the horizontal axis indicates a distance in a peripheral direction of the piezoelectric element and the vertical axis indicates a magnitude of a vibration amplitude. FIG. 15A shows a standing wave vibration generated in the electrodes for the A phase in its left half and shows a standing wave vibration generated in the electrodes for the B phase in its right half.
FIG. 15C shows a vibration amplitude of a travelling wave in which an A phase standing wave and a B phase standing wave are composited. Since the amplitude of the travelling wave is nonuniform in the peripheral direction, loci of rotational movements generated on the surface of the elastic member 10 are different as shown in FIG. 15B.
When there is unevenness in the vibration amplitude generated by the piezoelectric element as described above, since slipping occurs between the frictional member 30 of the elastic member 40 and the rotor 60, vibration energy cannot be used efficiently as driving energy. Therefore, unevenness in the vibration amplitude is not preferable.
There might be other factors that cause unevenness in the vibration amplitude generated by the piezoelectric element. Polarization processing of the piezoelectric element is performed by applying a voltage to the parts between the electrode patterns formed on both the sides of the piezoelectric element. Since an electric field at this point does not act on non-electrode portions as shown in FIG. 16, if the electrode patterns on both the sides of the piezoelectric element are different, a direction of an electric field applied to the parts between the electrode patterns becomes nonuniform and unevenness also occurs in a polarization direction. If unevenness exists in the polarization direction, since an elastic modulus of the piezoelectric element varies, unevenness also occurs in a vibration generated when a vibration wave driving apparatus is driven.
Therefore, in order to raise driving efficiency of the vibration wave driving apparatus, it is considered that there is still room for improvement.